Hi Yuval,

Thank you for your feedback.

> This will> likely link it to the receiver's other coins eventually, and
certainly
> links it to the receiver's subsequent transactions.

Coin control and labels can be used to avoid this. Consolidation of inputs
is often bad for privacy and makes silent payments, coinjoin etc. useless
in some cases however the user has the choice to select coins manually
while transacting. In payjoin, users can't do much about it. They have to
share UTXOs in response to the original PSBT along with the address to
receive bitcoin.

> In the payjoin setting, the receiver is
> using coinswap in that manner, then as a payjoin receiver they can
> elect to only use coinswapped coins as contributed inputs to payjoin
> transactions.

It could be a workaround or temporary fix for this problem. However, if
swapped coins are used in transactions, octojoin could be a better solution
which doesn't require any inputs from the recipient.

> I'm not sure what you mean by "the recipient would never doubt it
> because it's a privacy tool", it sounds to me like this is mainly a
> criticism of the UX of payjoin supporting wallets, or of wallets in
> general for not educating users that privacy is not a binary thing?

The recipient would never doubt a sender who insists on using payjoin and
not interested in a normal bitcoin transaction. They would not know the
intentions of the sender before payjoin.

> Note that in all of these specifications of payjoin UTXO probing is
> not costless since the sender must send a fully signed transaction in
> order to learn such a UTXO, and this transaction although not
> confirmed still imposes a fee cost on the sender if broadcast (even if
> it is replaced).

It was costless in the demo which could be fixed by bullbitcoin. However,
an attacker with a budget and some motivation can always spy on your wallet
using payjoin. Things become even easier with automated payment systems
such as BTCPay Server.

/dev/fd0
floppy disk guy

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